1. Download the Navionics plugin from the GpsGate Server Beta plugin repository "beta.gpsgate.com"
You do this from SiteAdmin->Plugins and select Get More Plugins. Change URL to http://beta.gpsgate.com Search for "Navionics" and click Install.
(You might also need the updated "CoreMap" plugin, please install it as above if it's requested).

2. Go to SiteAdmin->Maps->Navionics.
Enter your Navionics map key in the field named navionics_key

3. Go to SiteAdmin->Applications and enable the plugin for the application(s) of your choice. You do that in Privileges and Roles->Enable/Disable Features, then select Maps->_UseMapPlugin->Navionics.